1. Field of the Invention
This invention relates to a method for patterning regions formed on semiconductor structures. In particular the invention relates to a process for plasma etching portions of layers of aluminum on a semiconductor structure using chlorine based plasma.
2. Prior Art
Continuing improvements in semiconductor fabrication technology have caused the dimensions of some integrated circuit structures to be limited by the dimensions of aluminum or other metal lines formed on the surface of the structure to interconnect various portions of the underlying structure. Because the reactions involved are not directional, isotropic etchants, such as chemical etchants, are not suitable for etching through thick metal lines while minimizing the reduction of metal line width. One technique which has been developed to allow more accurate control of dimensions is etching of semiconductor structures with gas plasma reactions. Various processes for plasma etching are now known and have been the subject for several patents. See, e.g., U.S. Pat. No. 3,615,956 entitled "Gas Plasma Vapor Etching Process"; U.S. Pat. No. 3,994,793 entitled "Reactive Ion Etching of Aluminum"; and U.S. Pat. No. 4,057,460 entitled "Plasma Etching Process."
Plasma etching processes have been developed which use chlorine based plasmas to remove aluminum with little or no undercutting of overlying protective materials, for example, photoresist. Unfortunately, after removing the aluminum such plasma processes also will etch any silicon or silicon compound, for example, silicon, silicon nitride or silicon dioxide, beneath the aluminum. Because in commercial processing of large quantities of integrated circuits some or all of the integrated circuits will be etched at least in some areas longer than required to remove the aluminum, and because the rate of removal of material by plasma etching is not necessarily uniform across even the surface of a single circuit being etched, a significant amount of etching of materials underlying the aluminum has been observed. For example, in some areas of an integrated circuit the aluminum will be completely removed in a shorter time, while in other areas a longer time will be required. Consequently in those areas where the aluminum is removed relatively quickly the plasma etching will remove underlying materials. This problem usually is most severe in areas of integrated circuits where metal connections make direct contact to underlying silicon, for example, the emitters of bipolar transistors or the sources or drains of MOS transistors. In these areas of an integrated circuit there are frequently PN junctions close to the surface of the silicon, and even shallow plasma etching of the silicon can expose and/or destroy the PN junction and ruin the function of the device.
It is frequently impractical to ensure that these areas of integrated circuits are always completely covered by unetched material, and therefore protected from the plasma. Accordingly, one object of this invention is to overcome the difficulty of using plasma etching for patterning aluminum layers on semiconductor structures. A further object of this invention is to provide a region of plasma etch resistant material over selected portions of any underlying material, to thereby protect this material from etching by the plasma. A further object of this invention is to permit preservation of regions of lower layers of metal when devices with multiple layers of metal are etched.